1. Field of the Invention
The invention relates to downhole equipment as used in the mining of subterranean sulfur deposits, oil and gas field operations and similar operations. More particularly, the invention relates to the protection of the pipes used in such techniques from corrosion.
2. Prior Art
The Frasch process is a method for the mining of a deep-lying sulfur deposit, often found in shallow salt domes. Very hot or "superheated" water is forced into the deposit. The heated water under pressure is pumped down the outer of three concentric pipes. The liquid sulfur, which is very pure, collects in a pool at the bottom of the piping. Compressed air is then forced down the inner pipe forming a froth of the molten sulfur, which is forced up through the middle concentric pipe to the surface. The liquid mixture of sulfur and water is discharged, for example, into bins and the sulfur is allowed to solidify, or into barges or ships for transportation to plants.
In the Frasch process of mining sulfur, the downhole piping of the Frasch process sulfur wells is exposed to highly corrosive mixtures of molten sulfur and superheated (fresh, brine or seawater) water. Hydrogen sulfide and other severe corrodents are also present, these compounds being dissolved in the caprock formation water which is in contact with the sulfur well casing. Due to operating in this corrosive environment, corrosion has always been recognized as a major problem in Frasch sulfur well operation. The corrosion is compounded in "sensitive" or weak sulfur wells. In these wells, the molten sulfur pool at the bottom of the well is so small that the corrosive formation water may be drawn into the interior piping of the sulfur wells. Sulfur well piping can, in weak wells, fail in a matter of days. Depending upon the type of failure and how much downhole equipment can be salvaged after a failure, the cost of a sulfur well failure can be several hundred thousand dollars. This cost does not include lost production while the well is being reworked or replaced.
Many schemes and methods have been tried over the years to combat sulfur well corrosion (primarily with the bottom joint sulfur line) with mixed results. The problem is caused by the extreme temperature and corrosive conditions at the sulfur-water interface. The various corrective or preventive schemes and methods include different coatings, anodic sheaths and basic material changes. Cement linings have been applied around the interior of sulfur lines and have been very successful but the cement lining technique cannot be used on exterior surfaces of the downhole piping. Teflon materials and baked phenolic coatings were found, over the years, to be resistant to sulfur well corrosion; however, such materials had drawbacks that prevented their widespread use. Teflon has physical properties that have made it difficult and uneconomic to apply to downhole equipment. Phenolics are notoriously brittle, and corrosion appears to concentrate in areas where the coating has been removed. Coatings have some peculiar problems. The time required for curing is prohibitive in many cases. Quick curing coatings fail to have adequate resistance to the chemical and/or temperature conditions found downhole. The application of these coatings on the rig often prohibits their use or the quality of the coating is insufficient to meet minimum requirements. When the coatings are applied on the pipe prior to delivery to the rig, they are subject to damage by tongs, rotary pipe slips and other rig tools used to set the pipe.
Carbon steel is the principal material of construction for oil field tubular goods. Such tubings can be obtained in various grades and strengths, however, they are still basically carbon steel and fare very poorly in the downhole environment of sulfur wells. More exotic metals have proven to yield better results, however, such metals also have their problems. The cost of such materials prohibits installation of great quantities in the sulfur wells. When a partial string of these more noble metals is attached to a less noble metal (carbon steel), a corrosion cell is formed and makes the design even less durable than the carbon steel alone. Non-conductive couplings have been developed and tested, however, total isolation of the noble section from the other sulfur well piping is difficult. The final problem with the exotic metals is their availability in tubular form.
To overcome the effect of such corrosion cells, different materials have been used to sheath the more noble metal bottom joints. Such materials are sacrificially corroded to provide protection for the rest of the string, particularly the carbon steel joint immediately above the noble metal bottom joints in the string - the corrosion cells concentrate in that joint. The most promising of these materials was aluminum. However, the corrosion of aluminum yields a corrosion product with a significant increase in volume. This increase has been enough to totally fill the annular space in the liner thus prohibiting further injection of mine water. Carbon steel anodes are now being used for this service. It is questionable how effective they are as there is little difference in the oxidation potential of the anode and the rest of the string. The other limiting factor in the cathodic protection method is the life of the sacrificial anode. When it has been reduced to a corrosion product, it no longer provides protection for the sulfur line.
Bunnell Plastics, Inc., "Heat-Shrinkable Tubing Teflon FEP", Bulletin 205R, (9/30/79), discloses heat-shrinkable tubing composed of TEFLON FEP. The tubing is stated to have a shrinkage of about 20 percent at 230.degree. F.
Bunnell Plastics, Inc., "Installation Instructions For Bunnell Extruded Roll Covers Of FEP Or PFA Teflon", two page bulletin. The bulletin teaches a method of heat-shrinking a tube of Teflon FEP or Teflon PFA onto a roller which is rotated on a lathe. Two opposing heat guns are advanced along the sleeve to heat-shrink it onto the rotating pipe. Some of the problems with this type of system are discussed below.
U.S. Pat. No. 2,963,045 (Canevari et al.) discloses coating a heated pipe with polymer oil, partially curing the polymer oil by the direct action of a flame in the presence of oxygen and applying a layer of asphalt at high temperatures which completely cures the first layer. The two external layers are applied to protect the metal pipe from corrosion.
U.S. Pat. No. 3,610,291 (Heslop et al.) teaches preparing a protective layer for a pipe joint. A tube of a dimensionally heat unstable material, e.g., polyethylene, which has an elastic memory is shrunk onto a tube layer of mastic positioned over the joint.
U.S. Pat. No. 3,877,490 (Tsubouchi et al.) teaches a steel pipe containing a corrosion-resistant plastic layer, an antifusion agent layer thereon and a protective plastic layer thereon. The protective and corrosion-resistant layers are separately extruded on to the other layers of the pipe, respectively.
U.S. Pat. No. 4,091,134 (Uemura et al.) discloses a metal pipe coated with a corrosion-protection layer, which contains polypropylene, styrene-butadiene copolymer, asphalt and a tackifier. The coating is applied by using a heat-melt process. A thermoplastic resin, e.g., polyethylene, can be coated on the first layer by means of extrusion, powdering or tape binding processes.
U.S. Pat. No. 4,199,010 (McGuth et al.) discloses ceramic inner layers for pipe joints. U.S. Pat. No. 4,213,486 (Samour et al.) discloses pipe externally coated with fused powdered epoxy resin which is then wrapped with a polyolefin sheet for corrosion protection.
U.S. Pat. No. 4,245,674 (Nakamura et al.) teaches the use of thermoshrinkable or thermoexpansive tubes made of various rubbers or plastics such as fluorocarbon polymer (see col. 2, lines 24 to 32). The entire inner wall and at least part of the outer wall are covered with one or two peelable protective covers. By a complicated procedure, the inner protective layer is removed in order to get the thermoshrinkable tube over the uncovered joint region of two abutting pipes which contain an outer corrosion proof lining. The thermoshrinkable tube is heat sealed to the pipe joint area.
U.S. Pat. No. 4,287,034 (Pieslak et al.) discloses a method of coating a pipe using certain heat-shrinkable tapes or sheets which are coated on one side with an adhesive.
U.S. Pat. No. 4,312,904 (Meyer) teaches a method of coating a pipe with a hydraulic-setting mass. U.S. Pat. No. 4,371,578 (Thompson) discloses certain heat-shrinkable material for wrapping around a pipe. The mating edges of the material are provided with metal clip elements. U.S. Pat. No. 4,379,009 (Shibata et al.) discloses a method of sealing flange joints of pipes by using certain heat-shrinkable films and a wrapping method.
U.S. Pat. No. 4,386,984 (Jervis) teaches heat-shrinkable end caps for protecting the end of cables. U.S. Pat. No. 4,406,721 (Hoffman) discloses a method of applying heat-shrinkable film to bottles and other articles.